Clamshell Casting

A popular manufacturing technique is clamshell casting, where liquid is poured into a cast and the cast is removed by a rotation once the liquid has hardened. We consider the case where the object to be manufactured is modeled by a polyhedron with combinatorial complexity n of arbitrary genus. The cast consists of exactly two parts and is removed by a rotation around a line in space. The following two problems are addressed: (1) Given a line of rotation l in space, we determine in O(nlog n) time whether there exists a partitioning of the cast into exactly two parts, such that one part can be rotated clockwise around l and the other part can be rotated counterclockwise around l without colliding with the interior of P or the cast. If the problem is restricted further, such a partitioning is only valid when no reflex edge or face of P is perpendicular to l, the algorithm runs in O(n) time. (2) An algorithm running in O(n ⁴log n) time is presented to find all the lines in space that allow a cast partitioning as described above. If we restrict the problem further and find all the lines in space that allow a cast partitioning as described above, such that no reflex edge or face of P is perpendicular to l, the algorithm’s running time becomes O(n ⁴ α(n)). All of the running times are shown to be almost optimal.     

Growth of AlN Films and Its Process Development for the Fabrication of Acoustic Devices and Micromachined Structures

AlN films were grown on silicon substrates by RF reactive magnetron sputtering. At high sputtering powers, (002) preferred orientation as well as Al-N absorption band becomes prominent. The surface roughness and grain size of sputtered films were found to increase with RF power. Surface acoustic wave (SAW) device has been made on the grown (002) oriented piezoelectric AlN film with interdigital transducer (IDT) electrodes spacing corresponding to a wavelength of 60 μm. The centre frequency of the SAW filter was found to be 84.304 MHz, which gives a phase velocity of 5058 m/s with an electromechanical coupling coefficient (K ²) of 0.34%. Low etch rate of AlN films were observed in doped TMAH solution. Three-dimensional suspended Cr/AlN/Cr/SiO₂ microstructures were also fabricated by wet chemical etching.     

In vitro biological and tribological properties of transparent magnesium aluminate (Spinel) and aluminum oxynitride (ALON<Superscript>®</Superscript>)

The purpose of this first generation investigation is to evaluate the in vitro cytotoxicity, cell-materials interactions and tribological performance of Spinel and ALON^® transparent ceramics for potential wear resistant load bearing implant applications. Besides their non-toxicity, the high surface energy of these ceramics significantly enhanced in vitro cell-materials interactions compared to bioinert commercially pure Ti as control. These transparent ceramics with high hardness in the range of 1334 and 1543 HV showed in vitro wear rate of the order of 10^?6 mm³ Nm^?1 against Al₂O₃ ball at a normal load of 20 N.     

The effect of ozonation on natural organic matter removal by alum coagulation

Natural organic matter (NOM) was extracted from a moderately colored, eutrophic surface water source (Forge Pond, Granby, MA), and fractionated into quasi-homogeneous fractions. Fulvic acid (FA) and hydrophilic neutrals (HN) were the two most abundant NOM fractions that were isolated. Adsorption affinity of the isolated NOM fractions on preformed aluminum hydroxide flocs increased with increase in specific organic charge of the fractions, except for the two most highly charged fractions, FA and hydrophilic acids (HAA), which showed less adsorption affinity than expected based on their specific organic charge. Prior ozonation of FA and HN fractions resulted in a decline and an increase, respectively, in their adsorption affinity on aluminum hydroxide surface. Prior ozonation of Forge Pond raw water resulted in a progressive decline in dissolved organic carbon (DOC) removal by alum coagulation with increase in ozone dose. It appeared that ozone applied to raw water reacted preferentially with the humic fraction of NOM, resulting in the detrimental effects of ozonation on subsequent NOM removal by alum coagulation being magnified. Forge Pond raw water was pre-coagulated to remove humic substances. Ozonation of the pre-coagulated water demonstrated the beneficial effects of ozonation on the removal of non-humic NOM through alum coagulation. A strategy for staged coagulation with intermediate ozonation was proposed for waters containing both humic and non-humic NOM for maximum DOC and specific UV absorbance at 254nm (SUVA) removal.     

Characterization of a Ballard MK5‐E Proton Exchange Membrane Fuel Cell Stack

We present the results of an experimental investigation of the energy balance of a Ballard MK5‐E proton exchange membrane fuel cell (PEMFC) stack. We have investigated the transient phenomena that occur during PEMFC stack warm‐up, under load switching, and when the PEMFC stack is connected to a DC/AC inverter. A simple and convenient model describing the polarization curve as a function of the temperature is presented and validated by our experimental data. We also present experimental results on the increase PEMFC stack performance as a function of the current density for different oxygen concentrations of the oxidant gas.